NEW DELHI: Delhi's air has again made headlines for the wrong reasons. The US embassy's pollution monitor at Chanakyapuri on Wednesday measured air quality at 'very unhealthy' levels, prompting a post in an international news portal warning against letting children outdoors for prolonged periods in the capital.

The embassy's website calculated the air quality index (AQI) at 248 at 8am and 260 at 6pm. The index is a measure of air quality on a progressively deteriorating scale of 0 to 500, based on fine particulate matter (PM2.5) in the air.

According to US standards, readings in the 201-300 range are termed 'very unhealthy', which could lead to "significant aggravation of heart or lung disease and... significant increase in respiratory effects". The advisory for this level asks people with heart or lung disease, older adults and children to "avoid all physical activity outdoors".

The embassy releases hourly AQI figures. Wednesday's levels suddenly made news when a blog in the Wall Street Journal website quoted the figures. Ironically, it came on a day when air pollution in the city was at its lowest since Diwali and lower than the corresponding period last year.

Met department and earth sciences ministry air monitors showed only a slight different AQI level on Wednesday — an average of 220 for the day — but pegged the air quality level as 'poor'. That's because the ministry has a more relaxed categorization.

The air quality had worsened to 'severe' right after Diwali, according to monitoring by System of Air Quality Weather Forecasting and Research (SAFAR) under the earth sciences ministry. Air quality is expected to again worsen by November 2, when dropping temperatures could trap polluted air over the capital.

Said Gufran Beig, chief project scientist at SAFAR, "This time, Diwali pollution was lower than last year and PM 2.5 levels were the lowest on Wednesday. On October 24, PM 2.5 concentration was 270 microgram per cubic metre while on Wednesday it was 142."

Nevertheless, the PM2.5 levels have been a good three to four times than the Indian National Ambient Air Quality Standard (NAAQS) and several times the World Health Organisation standard of 25 microgram per cubic metre, exposing people to unhealthy air.

While SAFAR already has an AQI system, the Central Pollution Control Board and Ministry of Environment, Forests and Climate Change recently launched a national AQI that's yet to be rolled out. CPCB has set a period of 45 days to get feedback and comments. There are, however, concerns about the reliability of CPCB's equipment, said a former board official who declined to be quoted.

Interestingly, the US embassy's air quality monitor in Beijing — a city Delhi is often compared with for bad air quality — showed a slightly higher AQI level of 265 at midnight (Wednesday-Thursday).

The US embassy's air quality monitor has been a source of much political heartburn in China. Prominently displayed, the monitor has attracted criticism from the Chinese government, as well as relentlessly kept the attention of Beijingers on the quality of air they were breathing.

Unlike in India where the US Embassy is quick to point out that the Indian government too conducts its own real-time air quality monitoring, in China, where the government did not get into the business of informing its citizens, the US Embassy's action was viewed as a direct political challenge.

Also, unlike in India, where the US embassy's monitor is situated in Chanakyapuri, in Beijing, the US Embassy has positioned air quality monitors in several parts of that huge city.